1. Field of the Invention
The present invention relates to a motor drive circuit.
2. Description of the Related Art
A counter electromotive voltage of a motor coil in a common DC motor rises with rise in rotation speed of the motor. Thus, a coil current flowing through the motor coil increases at starting of the motor and then decreases, as illustrated in FIG. 4, for example. However, even during a steady period after the rotation speed of the motor has become constant, for example, if the motor is locked, the rotation speed of the motor is decreased, which may result in a great coil current flow. Thus, a motor drive circuit may be provided with a current limit circuit configured to compare a current value of the coil current with a predetermined current value and cause the current value of the coil current to be lower than or equal to the predetermined current value (See Japanese Patent Laid-Open Publication No. 2005-94925, for example). That is, if the current value of the coil current reaches the predetermined current value (hereinafter referred to as a current limit value), the motor drive circuit drives the motor coil so that the current value of the coil current is smaller than or equal to the current limit value, for example.
If the current limit value is determined on the basis of a current value for flowing during a steady period, generally the current value of the coil current results in the current limit value at starting. In this case, since the coil current is controlled so that the current value thereof is smaller than or equal to the current limit value, torque required for rotating the motor cannot be ensured, resulting that the motor might not rotate.
Thus, a microcomputer may be controlled so that the current limit value is set high while the motor is being started and is set low in a steady period, for example. In this case, however, the microcomputer has to change the current limit value every time the motor is started, which causes such a problem that the microcomputer is put under a load.